Albert Barnes Commentary Deuteronomy 6:8-9

"And thou shalt bind them for a sign upon thy hand, and they shall be for frontlets between thine eyes. And thou shalt write them upon the door-posts of thy house, and upon thy gates." — Deuteronomy 6:8-9 (ASV)

By adopting and regulating customary usages (for example, Egyptian ones), Moses provides both a check on superstition and a means of keeping the Divine Law in memory. Regarding the "frontlets" and "phylacteries" of the New Testament (Matthew 23:5), see Exodus 13:16.

The Jewish practice of the mezuzah is based on Deuteronomy 6:9 and Deuteronomy 11:20.

This word properly means door-post, which is how the term is translated in Deuteronomy 6:9 and 11:20, as well as in Exodus 12:7, 12:22, Exodus 21:6, and elsewhere. Among the Jews, however, it is the name given to a square piece of parchment inscribed with Deuteronomy 6:4-9 and Deuteronomy 11:13-21. This parchment is then rolled up in a small cylinder of wood or metal and affixed to the right-hand post of every door in a Jewish house.

The pious Jew touches the mezuzah on each occasion of passing, or kisses his finger, and speaks Psalms 121:8 in the Hebrew language.